Photodetector for weak light having charge reset means

ABSTRACT

The present invention provides a photodetector for weak light that can release charge from the circuit without increasing capacity; the present invention being characterized by a photodetector for weak light that has a charge release means which permits the photodetector for weak light of the present invention to avoid increasing its capacity because it does not require a reset transistor integrated with the photodector, rather the charge reset means of the present invention lowers mounting capacity by removing itself from the circuit after it releases the charge.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photodetector for weak light, andmore specifically a photodetector for weak light that has charge resetmeans. The charge reset means can release charge accumulated on acircuit, and it can reduce mounting capacity by being removed from thecircuit after it releases the charge.

2. Description of the Related Art

It is expected that optical communication and information processingtechnologies taking advantage of quantum properties such as quantumcryptography and quantum teleportation will further advance in thefuture. These technologies are expected to be introduced into theinfrastructure of information and telecommunication technology.Considering the present affinity of quantum communication experimentsfor techniques for generating, controlling, and detecting a photoncorrelation pair using a light in a 1.5 μm band that is a fiber low losswavelength band will be important for future communication. Detection ofa single photon is a basic technique for such communicationtechnologies. A photodetector for weak light capable of detecting asingle photon is a device which is not only necessary for suchcommunication technologies but also important for inspection of quantumoptics phenomena other than communication purposes.

At present, the device which detects a light in the 1.5 μm band is a PINphotodiode or an avalanche photodiode mainly containing InGaAs. Thedetector used in actual experiments for quantum cryptographic keydistribution is an APD in Geiger mode.

Required performances for the 1.5 μm band photodetector in a futurequantum communication field, include: high quantum efficiency (80percents or more), low error rate, high response rate, and capabilityfor discriminating the number of photons. If the APD is used as thedetector, it is difficult to satisfy these requirements. The reason isas follows. Since the APD in Geiger mode is intended to improve thesensitivity of the detector, the APD loses information on the number ofphotons at the peak value of the detector, and thus cannot discriminatethe number of photons such as one photon or two photons. In addition, ifthe sensitivity of the APD is improved, the error rate is increased,accordingly. For example, if a dark count of 0.1 percent is allowed, thequantum efficiency of the APD in Geiger mode is about 32 percents.

In recent years, a photodetector capable of measuring the number ofphotons of light in a 1.5 μm band using a bolometer made of asuperconductor has been developed (see Non-Patent Literature of A. J.Miller, S. Woo, J. M. Martinis, and A. V. Sergienko: “QCMC'02 (2002)”).This photodetector is required to be cooled down to 100 millikelvins(100 mK). In principle, the photodetector does not operate if it is notcooled to a deep cryogenic temperature. Therefore, it is considereddifficult to utilize this photodetector for communication purposes. Inaddition, the photodetector has disadvantages such as low couplingefficiency with respect to a fiber, and low quantum efficiency,evaluated at 20 percent.

SUMMARY OF THE INVENTION

Electrical charge accumulates on the circuit of a photodetector for weaklight. When the charge continuously accumulates, the output of thecircuit will be saturated and then the circuit cannot detect the signallight. Therefore, it is necessary to release the accumulated charge fromthe circuit before such a situation happens. To this end, a resettransistor is normally incorporated in the circuit. The reset transistorreturns the charge in the integrating read circuit to a ground level ora certain level. However, if the reset transistor is incorporated in theintegrating read circuit, the input capacitance of the circuit isincreased. When the input capacitance is increased, the accuracy of theoutput signal of the circuit is deteriorated and the performance of thephotodetector is deteriorated. It is an object of the present inventionto provide a photodetector for weak light that can release charge fromthe circuit without increasing capacity.

To attain the above object, the present invention is characterized by aphotodetector for weak light that has a charge release means. Thephotodetector for weak light of the present invention can avoidincreasing its capacity because it does not equip the reset transistorin conjunction with the photodetector. The charge reset means of thepresent invention can lower mounting capacity by removing itself fromthe circuit after it releases the charge.

According to a first aspect of the present invention, there is provideda photodetector for weak light comprising:

a substrate comprising an integrating read circuit that includes lightdetection means and a field effect transistor (FET); and

charge reset means for releasing charges accumulated in the integratingread circuit, wherein

the light detection means is one of a PIN photodiode or an avalanchephotodiode (APD), and

the charge reset means comprising: a rotation mechanism; a probe rotatedwhen the rotation mechanism rotates; and a connection section thatconnects the probe to an output.

According to the first aspect of the invention, it is preferable thatthe FET is a GaAs JFET.

It is also preferable that the integrating read circuit is one of acapacitive trans-impedance amplifier (CTIA) circuit or a chargeintegrating amplifier (CIA) circuit.

In the CTIA embodiment, it is also preferable that the integrating readcircuit comprises:

a field effect transistor (FET);

the light detection means connected to a gate electrode of the FET;

a capacitor connected to the gate electrode of the FET;

a resistor connected to a source electrode of the FET; and

an operational amplifier connected to the source electrode of the FET.

It is also preferable that a capacity of the capacitor is 0.01 to 1 picofarad (pF).

It is also preferable that the capacitor has an area of 0.1 to tensquare millimeter (mm²), and a thickness of 0.1 to 0.5 millimeter (mm).

It is also preferable that the dielectric that constitutes the capacitorcontains quartz glass.

In the CIA embodiment, it is preferable the integrating read circuitcomprises:

a field effect transistor (FET);

the light detection means connected to a gate electrode of the FET;

a first resistor connected to a source electrode of the FET;

an operational amplifier connected to the source electrode of the FETthrough the first resistor; and

a second resistor that connects the negative input terminal of anoperational amplifier to the output terminal of the operationalamplifier.

It is also preferable that the resistance value of the resistors is 100kilo ohms (kΩ) to 30 mega ohms (MΩ).

It is also preferable that the charge reset means comprises:

an ascent and descent mechanism;

a probe that rises and falls by the ascent and descent mechanism; and

a connection section that connects the probe to an output.

It is also preferable that the probe can be brought into contact withthe integrating read circuit, and can be separated from the integratingread circuit by 1 millimeter (mm) or more.

Generally speaking, a photodetector for weak light is small. Thus, asmall transistor is commonly equipped in the circuit of a photodetectorfor weak light. The inventors found that such a photodetector for weaklight has a capacitance from a reset transistor. Though thephotodetector for weak light of the present invention is larger than theconventional weak light photodetectors because it has the charge resetmeans, the present invention can release charge without increasingcapacity.

The present invention can provide the photodetector for weak light andthe detection system with a high enough sensitivity to be able tomeasure the number of photons. The present invention can particularlyprovide a photodetector for weak light which sufficiently operates attemperatures as low as 4.2 kelvins (4.2K). The previous photon numberresolving photodetector must be cooled to 100 millikelvins (100 mK). Thephotodetector for weak light according to the present invention, bycontrast, can measure the number of photons in a temperature environmentthat can be obtained by cooling the device using only liquid helium. Thephotodetector for weak light according to the present invention is,therefore, applicable to various devices and easy to utilize. Thephotodetector for weak light according to the present invention does notuse nonlinear amplification means. Therefore, the photodetector for weaklight according to the present invention can measure the number ofphotons without losing information on the number of photons.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a weak light detection system according tothe present invention;

FIG. 2 is a schematic diagram which depicts an embodiment of the weaklight detection system according to the present invention;

FIGS. 3A and 3B are schematic diagrams which depict another embodimentof the weak light detection system according to the present invention,wherein FIG. 3A is a top view of the weak light detection system, andFIG. 3B is a cross-sectional view taken along a line A—A of FIG. 3A;

FIG. 4 is a block diagram of a CTIA circuit according to the presentinvention;

FIG. 5 is a block diagram of a CTA circuit according to the presentinvention;

FIGS. 6A and 6B depict noise spectrums, wherein FIG. 6A depicts a noisespectrum if an ultraviolet transmittable substrate is used, and FIG. 6Bdepicts a noise spectrum of a substrate that does not transmit UV light;

FIGS. 7A to 7C are schematic diagram of a charge reset means accordingto the present invention, wherein FIG. 7A is a schematic diagram whichdepicts an example of the charge reset means which includes a rotationmechanism, FIG. 7B is a schematic diagram which depicts an example ofthe charge reset means which includes an ascent and descent mechanism,and FIG. 7C is a schematic diagram which depicts an example of thecharge reset means which includes a diagonally provided ascent anddescent mechanism; and

FIG. 8 is a schematic diagram for explaining manufacturing stepsaccording to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

A first embodiment of a photodetector for weak light will be describedwith reference to FIG. 1. As shown in FIG. 1, the photodetector for weaklight includes a light detection means 3 which receives and detects alight from a light source 2, an integrating read circuit 4, and asubstrate 5 on which the light detection means 3 and the integratingread circuit 4 are integrated. The integrating read circuit 4 preferablyincludes a field effect transistor (FET) 6. The photodetector for weaklight according to the present invention may include an output means 7which outputs a voltage or a current output from the integrating readcircuit 4 outside of the substrate 5. The photodetector for weak lightaccording to the present invention includes a charge reset means 8 whichgrounds the integrating read circuit.

Light Source

As the light source 2, any well-known light source may be applicable. Anoutput portion of the light source 2 is preferably an optical fiber,more preferably a single mode optical fiber, most preferably a standardsingle mode optical fiber.

Light Detection Means

The light detection means is a means for detecting light. The lightdetection means 3 include photodetectors such as a PIN photodiode and anavalanche photodiode. The PIN photodiode is preferably an InGaAs PINphotodiode. Preferably, the APD in Geiger mode is not used. The lightdetection means 3 is preferably positioned in the center of thesubstrate 5.

Integrating Read Circuit

The integrating read circuit 4 is, for example, a circuit which detectsa light by integrated charges of the optically pumped photodetector in agate electrode of the FET 6, and measuring a voltage generated byintegrating. As will be described later, examples of the integratingread circuit 4 include a capacitive trans-impedance amplifier (CTIA)circuit and a charge integrating amplifier (CIA) circuit. Examples ofthe integrating read circuit 4 according to the present inventioninclude an integrating read circuit 4 that includes a FET 6.

Substrate

The substrate 5 utilizable for the photodetector for weak lightaccording to the present invention is not limited as long as thesubstrate 5 can be equipped with the integrating read circuit 4 andallows the circuit 4 to function. Examples of the substrate 5 that areutilizable for the photodetector for weak light according to the presentinvention include ultraviolet (UV) transmittable substrates such assubstrate consisting of calcium fluoride (CaF₂), silicon dioxide, quartzglass (a-SiO₂), crystallized quartz (crystal quartz), sapphire, aluminumoxide, magnesium oxide, lithium niobate (LiNbO₃), strontium titanate(SrTiO₃), magnesium fluoride (MgF₂), potassium bromide (KBr), andfluororesin. The reason for the use of the UV transparent substrate isto reduce noise deriving from dielectric polarization. By using the UVtransparent substrate, even if the substrate 5 is cooled cryogenicallyto about 4.2K, breakage of a wiring pattern can be prevented. Thesubstrate 5 that constitutes the photodetector for weak light accordingto the present invention preferably has a high purity. A molarconcentration of the substrate 5 is preferably 99.99 percents or more,more preferably 99.999 percents or more, far more preferably 99.9999percents or more, most preferably 99.99999 percents or more.

A shape of the substrate 5 is not limited. The substrate 5 is normallyflat rectangular, preferably flat square. As a size of the substrate 5,a length of one side is, for example, 5 to 30 millimeters, preferably 10to 20 millimeters, more preferably 12 to 18 millimeters, most preferably15 millimeters. An area of the substrate 5 is preferably 25 to 900 mm²,more preferably 100 to 400 mm², far more preferably 150 to 300 mm², mostpreferably 225 mm². If the substrate 5 is too small in size, then thelight detection means 3, the integrating read circuit 4, and the likecannot be mounted on the substrate 5, and it is difficult to manufacturethe photodetector for weak light. If the substrate 5 is too large, thephotodetector is made large, accordingly. Electrodes are formed on thesubstrate 5 by, for example, photolithography.

Output Means

The output means is a means for outputting the output of thephotodetector for weak light. The output means 7 utilizable for thephotodetector for weak light according to the present invention is notlimited as long as the output means 7 can transmit the output of thephotodetector for weak light to the weak light detection system.Examples of the output means 7 include a copper wire that connects thephotodetector for weak light to the outside, and an amplifier.

Weak Light Detection System

Examples of the weak light detection system 9 according to the presentinvention include the weak light detection system which includes thephotodetector for weak light, and which also includes a conversion means10 for converting information output from the output means 7 intoinformation on the number of photons or the like, and a display means 11for displaying the information on the number of photons obtained by theconversion means 10.

Conversion Means

The conversion means is a means for decoding the voltage or the likeoutput from the photodetector for weak light to information on thenumber of photons. The conversion means 10 in the weak light detectionsystem according to the present invention is not limited as long as theconversion means 10 can decode the voltage or the like output from thephotodetector for weak light to information on the number of photons.

Display Means

The display means is a means for displaying the information on thenumber of photons detected by the photodetector for weak light. Thedisplay means 11 in the weak light detection system according to thepresent invention is not limited as long as the display means 11 candisplay the information on the number of photons detected by thephotodetector for weak light. For examples, a well-known display meanssuch as an oscilloscope, a computer display, or a CRT can be used as thedisplay means 11.

Function of Weak Light Detection System

In the weak light detection system according to the present invention,the light detection means 3 first detects a light emitted from the lightsource 2. Information on the light detected by the light detection means3 is converted into, for example, voltage information by the integratingread circuit 4. The information on the light converted into the voltageor the like is output by the output means 7. The voltage output by theoutput means 7 is converted into information on the number of photons orthe like by the conversion means 10 in the weak light detection systemaccording to the present invention. The information on the number ofphotons or the like converted from the voltage by the conversion means10 is displayed on the display means 11 such as a display.

Example of the System

FIG. 2 depicts one example of the weak light detection system accordingto the present invention. As shown in FIG. 2, the weak light detectionsystem includes the substrate 5, on which the light detection means 3,the integrating read circuit 4, and the like are mounted, a substratesupport section 21 which supports the substrate 5, a support column 22which supports the substrate support section 21 so as to facilitate thesubstrate support section 21 to move in parallel, and a work surface 23which supports the support column 22. Such a weak light detection systemcan adjust a position of the light detection means 3 on the substrate 5,and can arrange the light detection means 3 at an appropriate position.

In such a weak light detection system, the work surface 23 is preferablydistant from the substrate 5. A distance between the work surface 23 andthe substrate 5 is preferably 1 millimeter or more, more preferably 3millimeters or more, far more preferably 5 millimeters or more, mostpreferably 1 centimeter or more. The reason is as follows. The substrate5 and the work surface 23 constitute one capacitor. If the distancebetween the substrate 5 and the work surface 23 that constitute thecapacitor is smaller, more charges are accumulated at a lower voltage.According to the present invention, a FET 6 which is damaged even byvery small charges is preferably used. Therefore, it is preferable notto accumulate more charges.

The shape of the substrate support section 21 is not limited as long asthe substrate support section 21 can mount the substrate 5 thereon.Preferably, the substrate support section 21 is composed by two convexportions and mounts the substrate 5 thereon at the two convex portions.This is because it is preferable that a contact portion between thesubstrate 5 and the substrate support section 21 is smaller. Examples ofthe convex portion include a prism shorter than the length of thesubstrate 5 to be mounted.

The weak light detection system according to this embodiment includes,for example, an input device which inputs an instruction for moving thesubstrate support section 21, a converter which converts the instructionfor moving the substrate support section 21 input to the input deviceinto electrical information, a transmission means for transmitting theinformation converted from the instruction by the converter to aninstruction section, and an actuator which moves the substrate supportsection 21 horizontally based on the information transmitted from thetransmission means to the instruction section. Namely, according to thisweak light detection system, the instruction for moving the substratesupport section 21 input to the input device is converted intoelectrical information by the converter, the electric information istransmitted to the instruction section by the transmitting means, andthe instruction section issues an instruction to the actuator, therebymoving the substrate support section 21 horizontally. In other words,the weak light detection system can adjust the position of the substrate5 by inputting positional information to the input means. According toanother preferable embodiment of the present invention, the substratesupport section 21 is vertically movable.

According to another preferable embodiment of the present invention, thesubstrate 5 is fixed, and the light source 2 is movable in, for example,a horizontal direction by light source moving means. Examples of thelight source moving means include means which grips the light source 2,and which can move the light source 2 according to input informationfrom the input means using an actuator or the like. The movablesubstrate is not always desirable for the following reason. If thesubstrate 5 is movable, a member that generates an electric field isundesirably located near the substrate 5.

FIG. 3 depicts another embodiment of the weak light detection systemaccording to the present invention. FIG. 3A is a top view of the system,and FIG. 3B is a cross-sectional view taken along ling A–A′ of FIG. 3A.As shown in FIG. 3A, this weak light detection system includes a jacket31 provided to be substantially concentric with the light detectionmeans 3, a substrate support section 32 provided in the jacket 31, andmounted on the substrate support section 32 a the substrate 5. Thesubstrate 5 is deformed by thermal expansion. If the substrate 5 isdeformed, the position of the light detection means 3 isdisadvantageously shifted from an optical axis. However, by using thisjacket 31 and providing the light detection means 3 at the center of thejacket 31, the deformation of the substrate 5 caused by thermalexpansion can be made isotropic from the light detection means 3,thereby making it possible to reduce the amount of shift in the lightdetection means 3.

Examples of a material of the jacket 31 include aluminum, brass, copper(e.g., pure copper), gold, silver, iron, and alloys thereof. Thematerial of the jacket 31 is preferably aluminum, brass, or copper(e.g., pure copper), more preferably aluminum or pure copper. Since thejacket 31 preferably, satisfactorily conducts heat to the chamber thatsurrounds the photodetector for weak light, a material having anexcellent heat conductivity is preferably used as the material of thejacket 31. In addition, since the jacket 31 is formed into a cylindricalshape or the like, a material having a high workability is preferablyused as the material of the jacket 31.

Examples of the jacket 31 include a cylindrical jacket. The size of thejacket 31 is not limited as long as the jacket 31 can contain thereinthe substrate 5. However, if the jacket 31 is too large, it is difficultto cool the substrate 5. The outside diameter of the jacket 31 is,therefore, preferably 10 millimeters to 50 centimeters, more preferably15 millimeters to 10 centimeters, far more preferably 25 to 50millimeters, most preferably 30 millimeters.

As shown in FIGS. 3A and 3B, the substrate support section 32 ispreferably divided into two portions 32 a and 32 b so that the twoportions 32 a and 32 b are equal in height and thereby hold thesubstrate 5 substantially parallel. It is preferable that the substrate5 is put at a high position to some extent for the same reason as thatstated above. For the stability of the substrate 5, at least a part ofthe substrate support section 32 preferably includes a stepped portion(32 a).

The height 33 of the substrate support section 32 at which the substratesupport section 32 supports the substrate (e.g., the height from thework surface, which extends horizontally and in which the support columnis buried, to the substrate 5) is preferably 1 millimeter or more and 10centimeters or less, more preferably 3 millimeters or more and 5centimeters or less, far more preferably 5 millimeters or more and 3centimeters or less, most preferably eight millimeters.

At least a part of the substrate support section 32 preferably includesthe stepped portion. This stepped portion enables stable holding of thesubstrate 5. The height of the stepped portion 32 b is not limited.Preferably the height of the stepped portion 32 b is 1 millimeter to 1centimeter, more preferably 2 millimeters.

According to another embodiment of the present invention, the substrate5 is fixed and the light source 2 can be moved in, for example, thehorizontal direction by the light source moving means. Examples of thelight source moving means include means which grip the light source 2,and which can move the light source 2 according to input informationfrom the input means using an actuator or the like. The movablesubstrate is not always desirable for the following reason. If thesubstrate 5 is movable, a member that generates an electrical field isdisadvantageously located near the substrate 3. According to anotherpreferred embodiment, the light source 2 can be moved by the samemechanism for moving the substrate support section 32 already statedabove.

Operation Description of Integrating Read Circuit

The integrating read circuit 4 according to the present inventiondetects a light by accumulating optically pumped charges in a feedbackcapacitor or a gate of an amplifier provided in a first stage of thecircuit 4, and measuring a generated voltage. FIG. 4 is a circuitdiagram which depicts an example of the integrating read circuit 4 thatincludes the CTIA, which is one constituent element of the integratingread circuit 4 according to the present invention.

CTIA Circuit

As shown in FIG. 4, examples of the integrating read circuit 4 thatincludes the CTIA circuit according to the present invention include theintegrating read circuit 4 which includes the FET 6, the light detectionmeans 3 connected to the gate electrode 6 a of the FET, a capacitor 41connected to the gate electrode 6 a of the FET, a resistor 42 connectedto a source 6 b of the FET 6, and an operational amplifier 43 connectedto a source electrode 6 b of the FET 6. One end (a positive inputterminal) of the operational amplifier 43 is grounded. An outputterminal of the operational amplifier 43 is connected to one end of thecapacitor 41, and a voltage is output from the circuit 4.

Field Effect Transistor (FET)

As the FET 6, a well-known FET operable at a low temperature can beused. The FET 6 is a kind of transistor called a “field effecttransistor”. Types of FET 6 are classified as either a junction FET or aMOS FET according to its structure, and further classified as either a Pchannel FET or an N channel FET corresponding to PNP and NPNtransistors, respectively.

According to the present invention, the FET 6 is preferably a junctionFET, more preferably the junction FET using GaAs (hereinafter, “GaAsJ-FET”). The GaAs J-FET performs a transistor operation at a temperatureof 4.2K or less, and is characteristically low in capacitance and low inleakage current. Therefore, the GaAs J-FET is effective in the highsensitivity, high resistance photodetector that operates cryogenically.Specifically, a GaAs J-FET manufactured by Sony Corporation can be used.

Capacitor

The capacitance of the capacitor 41 is adjusted according to a quantityof incident light. The capacitance of the capacitor 41 is preferably0.01 to 1 picofarad (pF).

The capacitor 41 which does not generate additional noise such as adielectric polarization noise is preferably used. Since thephotodetector for weak light according to the present invention operatescryogenically, the capacitor 41 is preferably small in fluctuation ofcapacitance relative to temperature change, and excellent in operationalstability at cryogenic temperatures. In addition, the capacitor 41preferably does not cause a leakage field or a leakage current. Fromthese viewpoints, a dielectric that constitutes the capacitor 41 has anarea of preferably 0.1 to 10 mm², more preferably 0.4 to 5 mm², far morepreferably 0.8 to 4 mm², most preferably 1 mm². The dielectric thatconstitutes the capacitor 41 has a thickness of preferably 0.1 to 0.5millimeter, more preferably 0.2 to 0.4 millimeter, most preferably 0.3millimeter. The material of the dielectric is preferably a UVtransparent material such as high purity quartz glass. By using such aUV transparent material, a capacitor 41 capable of suppressing thedielectric polarization noise can be obtained. The electrode of thecapacitor 41 is preferably a high strength conductive adhesive. Namely,by using a high strength conductive adhesive as the electrode,occurrence of the noise on an interface can be avoided.

More specifically, the capacitor 41 having high purity quartz glass,which has an electrode surface of 1 millimeter by 1 millimeter, andwhich has a thickness of 0.3 millimeter, coated with a high strengthconductive adhesive is used. The capacitance of this capacitor 41 is0.03 pico farad.

Resistance

A well-known resistor can be used as the resistor 42. The resistor 42has a resistance value of preferably 100 kilo ohms to 30 mega ohms, morepreferably 5 to 15 mega ohms, far more preferably eight to 12 mega ohms,most preferably 10 mega ohms. According to the present invention, aresistor 42 having such a high resistance value is used.

Operational Amplifier

A well-known operational amplifier can be used as the operationalamplifier 43 according to the present invention. The operationalamplifier 43 is preferably a low noise amplifier. Specifically, a lownoise amplifier manufactured by Analog Devices such as “OP27” (productname) can be used as the operational amplifier 43.

Operation of CTIA Circuit

The light detection means 3 receives a light. Then, the light detectionmeans 3 outputs electrons pumped by photons. The output electrons areaccumulated in the gate electrode 6 a of the FET 6. A voltage with agate voltage rise V_(out) according to the following Equation 1 isoutput from the operational amplifier 43.

Equation  1:   $V_{out} = {- \frac{Q}{C_{f}}}$

In the Equations, Q denotes generated charges, GM denotes a sourcefollower gain, and C_(f) denotes a feedback capacitance.

An output noise can be expressed by the following Equation 2.

Equation  2:  ${Noise}_{out} = {- \frac{( {C_{input} + C_{f}} )V_{noise}}{{GMC}_{f}}}$

In the equation, C_(input) denotes a gate input capacitance+a floatingcapacitance, and V_(noise) denotes channel noise of the FET 6.Accordingly, a S/N ratio of the system is expressed by the followingEquation 3.

${{{Equation}\mspace{14mu} 3\text{:}}\mspace{14mu}\therefore\frac{S}{N}} = \frac{GMQ}{( {C_{input} + C_{f}} )V_{noise}}$

Charge release is carried out preferably before an output voltage issaturated. To release charges, a reset FET or the charge reset means tobe described later may be used.

In the weak light detection system according to the present invention, afactor that determines the S/N ratio is the output voltage of the FET 6.The ratio of voltage at which a single electron is generated to noisecharacteristic of the FET 6 determines whether the single electron canbe counted. It is important, therefore, to reduce the noise of the FET 6for the system according to the present invention.

As will be described later, according to the present invention, thephotodetector for weak light is cooled to a low temperature such as 4.2Kby, for example, a cryostat, thereby making it possible to reduce thenoise. If the temperature is about 4.2K, the photodetector can be cooledrelatively easily by using liquid helium or the like. A well-knowncryostat can be used as the cryostat. The cryostat preferably cools thephotodetector by liquid helium. By cooling the photodetector for weaklight to 4.2K, the following advantages can be obtained: reduction ofthermal noise, reduction of hopping current, reduction of dielectricpolarization noise, and reduction of the gate leakage current of the FET6. The thermal noise power is proportional to an element temperature.Therefore, the lower the temperature of the photodetector, the lower thenoise. A mobility of the hopping current which may possibly be a darkcurrent of the detector is proportional to exp(−U/kT) (U: barrierheight). Therefore, the lower the temperature of the photodetector, thelower the hopping current. The dielectric polarization noise isexpressed by the following Equation 4.I _(n) ²=4kTωC″V _(n) ² =I _(n) ²/(ωC _(f))²  Equation 4

In Equation 4, ω denotes an angular frequency and C″ denotes a complexcomponent of the capacitance.

The dielectric polarization noise corresponds to a fluctuation of adielectric loss. Equation 3 indicates that the dielectric polarizationnoise can be reduced by decreasing the element temperature, and byselecting a matter having a low dielectric loss. The gate leakagecurrent of the FET 6 can be explained by the same physical process asthat of the hopping current.

CIA Circuit

As shown in FIG. 5, examples of the integrating read circuit 4, thatincludes the CIA according to the present invention, include theintegrating read current which includes the FET 6, the light detectionmeans 3 connected to the gate electrode 6 a of the FET 6, a resistor 44connected to the source electrode 6 b of the FET 6, the operationalamplifier 43 connected to the source electrode 6 b of the FET 6 throughthe resistor 45, and a resistor 46 which connects a negative inputterminal of the operational amplifier 43 to an output terminal thereof.One end (the positive input terminal) of the operational amplifier 43 isgrounded. A voltage is output from the output terminal of theoperational amplifier 43. Since the same constituent elements of the CIAcircuit as those of the CTIA circuit can be used, they will not bedescribed herein.

Output and the Like of CIA Circuit

The output and the like of the CIA circuit are expressed by thefollowing Equation 5.

Equation  5:  ${V_{out} = {{- {GM}}\frac{Q}{C_{input}}}},{{Noise}_{out} = {- V_{noise}}},{{{and}\therefore\frac{S}{N}} = \frac{GMQ}{C_{input}V_{noise}}}$

In Equation 5, Q denotes generated electric charges, V_(noise) denotesthe channel noise of the FET 6, GM denotes a source follower gain,C_(input) denotes a gate input capacitance+a floating capacitance, andC_(f) denotes a feedback capacitance.

Preferable Example of Substrate and Verification thereof

As a method for measuring impurities in the dielectric, a fluorescenceanalysis method is known. If the dielectric transmits ultraviolet rays,this means that a concentration of impurities contained in thedielectric is low. If impurities are mixed into the dielectric, noisesuch as the dielectric polarization noise occurs and the noise adverselyinfluences the weak light detection system. Preferably, therefore, a UVtransparent substrate is used as the substrate 5 that constitutes thephotodetector for weak light.

FIG. 6A depicts a noise spectrum if the UV transparent substrate isused, and FIG. 6B depicts a noise spectrum if the UV transmissionsubstrate is not used. The noise is measured by R9211BFET Servo-Analyzermanufactured by Advantest Corporation. The system shown in FIGS. 3A and3B is used as the weak light detection system during the measurement ofthe noise spectrum. The comparison between FIGS. 6A and 6B shows that ifthe UV transparent substrate is not used, many noises resulting fromimpurities are observed in a frequency region of about—2 to 70 hertz.This indicates that the UV transparent substrate is preferably includedin the photodetector for weak light.

Charge Reset Means

While the integrating read circuit 4 operates accurately, electricalcharges can be accumulated in the circuit 4. However, if the charges arecontinuously accumulated, the output of the circuit 4 is saturated andthe circuit 4 cannot detect the signal light. Before this state occurs,it is necessary to release the accumulated charges from the circuit 4.To this end, a reset transistor is normally incorporated in the circuit4. The reset transistor returns the charges in the integrating readcircuit 4 to a ground level or a certain level. However, if the resettransistor is incorporated in the integrating read circuit 4, the inputcapacitance of the circuit 4 is increased. If the input capacitance isincreased, the accuracy of the output signal of the circuit 4 isdeteriorated and thus the performance of the photodetector isdeteriorated.

According to the present invention, therefore, it is preferable torelease the accumulated charges from the integrating read circuit 4without using a reset transistor. Namely, the charge reset means, e.g.,a mechanical switch, releases the charges from the integrating readcircuit 4 by physically contacting a probe of the mechanical switch withthe circuit 4 if the charges are accumulated in the circuit 4. Ifcharges are to be accumulated in the circuit 4, the probe of themechanical switch is physically separated from the circuit 4 by, forexample, 1 millimeter or more. By doing so, the influence of themechanical switch on the input capacitance of the circuit 4 can belessened to an ignorable level. If a distance between the circuit 4 andthe probe of the mechanical switch is wider, the influence of themechanical switch on the input capacitance is smaller. Therefore, thedistance between the integrating read circuit 4 and the probe ispreferably 1 millimeter or more, more preferably 1.5 millimeters ormore, most preferably 2 millimeters or more.

The charge reset means according to the present invention is connectedto the integrating read circuit 4 when the charges are to be released,and thereby the charge reset means releases the charges accumulated inthe circuit 4. Once the release of the charges is finished, the chargereset means can be separated farther from the circuit 4. The mountingcapacity can be thereby reduced.

FIGS. 7A to 7C depict examples of the charge reset means according tothe present invention. The charge reset means is not limited as long asthe charge reset means can contact with the integrating read circuit 4,drop the voltage of the circuit 4 to the ground level or the certainlevel, and thereby release the charges from the circuit 4. A well-knowncharge reset means can be used.

FIG. 7A depicts an example of the charge reset means which includes arotation mechanism. As shown in FIG. 7A, this charge reset meansincludes the rotation mechanism 51, a probe 52 rotated when the rotationmechanism 51 rotates, and a connection section 53 which connects theprobe 52 to the outside and which consists of a copper wire or the like.If an instruction for releasing the charges from the circuit 4 istransmitted from the outside, then the rotation mechanism 51 rotates ina direction “a” shown in FIG. 7A, and the probe 52 comes in contact withthe circuit 4 on the substrate 5. When the probe 52 contacts with thecircuit 4, the charges accumulated in the circuit 4 are released to theoutside through the probe 52 and the connection section 53. If aninstruction for separating the probe 52 from the circuit 4 istransmitted, the rotation mechanism 51 rotates in a direction “b” shownin FIG. 7A. The probe which has been in contact with the circuit 4 isseparated from the circuit 4, accordingly. The contact and separation ofthe probe 52 may be carried out in response to an input from an externalinput device, automatically according to the amount of chargesaccumulated in the circuit 4, or at predetermined time intervals.

FIG. 7B depicts another example of the charge reset means which includesan ascent and descent mechanism. As shown in FIG. 7B, this charge resetmeans includes an ascent and descent mechanism 55, a probe 52 caused torise and fall by the ascent and descent mechanism 55, and a connectionsection 53 which connects the probe 52 to the outside, and whichconsists of the copper wire or the like. If an instruction for releasingthe charges from the circuit 4 is transmitted from the outside, then theascent and descent mechanism 54 operates to cause the probe 52 to fallin a direction “a” shown in FIG. 7B, and the probe 52 comes in contactwith the circuit 4 on the substrate 5. When the probe 52 contacts withthe circuit 4, the charges accumulated in the circuit 4 are released tothe outside through the probe 52 and the connection section 53. If aninstruction for separating the probe 52 from the circuit 4 istransmitted, the ascent and descent mechanism 54 operates to cause theprobe 52 to rise in a direction “b” shown in FIG. 7B. The probe 52 which

FIG. 7B depicts another example of the charge reset means which includesan ascent and descent mechanism. As shown in FIG. 7B, this charge resetmeans includes an ascent and descent mechanism 54, a probe 52 caused torise and fall by the ascent and descent mechanism 54, and a connectionsection 53 which connects the probe 52 to the outside, and whichconsists of the copper wire or the like. If an instruction for releasingthe charges from the circuit 4 is transmitted from the outside, then theascent and descent mechanism 54 operates to cause the probe 52 to fallin a direction “a” shown in FIG. 7B, and the probe 52 comes in contactwith the circuit 4 on the substrate 5. When the probe 52 contacts withthe circuit 4, the charges accumulated in the circuit 4 are released tothe outside through the probe 52 and the connection section 53. If aninstruction for separating the probe 52 from the circuit 4 istransmitted, the ascent and descent mechanism 54 operates to cause theprobe 52 to rise in a direction “b” shown in FIG. 7B. The probe 52 whichhas been in contact with the circuit 4 is separated from the circuit 4,accordingly. The contact and separation of the probe 52 may be carriedout in response to the input from the external input device,automatically according to the amount of charges accumulated in thecircuit 4, or at predetermined time intervals.

FIG. 7C depicts yet another example of the charge reset means whichincludes a diagonally provided ascent and descent mechanism 54.Differently from FIG. 7B, it is unnecessary to provide this ascent anddescent mechanism 54 vertically. The ascent and descent mechanism 54 maybe provided diagonally. It is preferable to diagonally provide themechanism 54 since the mechanism 54 and the charge reset means can bedistanced from the circuit 4.

Manufacturing Method

It is necessary to connect the light detection means 3 to the FET 6 sothat the photodetector for weak light operates. However, thephotodetector for weak light includes the FET (e.g., GaAs J-FET) 6 whichis destroyed even by a low serge voltage. Therefore, if a solderingoperation is carried out while the gate electrode 6 a of the FET 6 isconnected to the light detection means 3, the gate electrode 6 a of theFET 6 is destroyed even by the remaining heat of a solder or the like.Namely, with the conventional manufacturing method, it is difficult tomanufacture the photodetector for weak light according to the presentinvention. A manufacturing method capable of manufacturing thephotodetector for weak light without destroying the FET 6 is desired.The destruction of the FET can be prevented by executing the followingsteps. The manufacturing method will be described with reference to FIG.8.

Light Detection Means Connection Step

Electrodes 61 to 67 are provided in peripheral portions of the substrate5 and portions slightly away from the center of the substrate 5,respectively. A connection port 68 for connecting the photodetector forweak light to an external device is prepared. The light detection means3 such as the APD is attached first to the substrate 5, and one end ofan electrode of the light detection means 3 is connected to theelectrode 61 provided in the peripheral portion of the substrate 5 (alight detection means connection step).

FET Electrode-to-Substrate Electrode Connection Step

The respective electrodes of the FET 6 are connected to the electrodeson the substrates 5 (in an FET electrode-to-substrate electrodeconnection step). In this FET electrode-to-substrate electrodeconnection step, the gate electrode 6 a of the FET 6 is connectedthrough electrode 62 to the electrode 63 provided in the peripheralportion of the substrate 5 by a copper wire or the like. The sourceelectrode 6 b and the drain electrode 6 c of the FET 6 are connected to,for example, the electrodes 66 and 67 provided in the peripheralportions of the substrate 5 through the electrodes 64 and 65 provided onthe substrate 5, respectively.

External Electrode Connection Step

The electrode 61 connected to one of the electrodes of the lightdetection means 3, and the electrodes 63, 66 and 67, which are connectedto the electrodes of the FET 6, and which are provided in the peripheralportions of the substrate 5, are connected to electrodes 69 to 72 on theconnection port 68 which is connectable to external terminals providedoutside of the photodetector for weak light, respectively (externalelectrode connection step). It is noted that when the photodetector forweak light operates, the photodetector is normally cooled to about 4.2K,but the connection port 68 is connected to, for example, electrodes at aroom temperatures.

Light Detection Means-to-Gate Connection Step

The electrode of the light detection means 3 such as the APD isconnected to the electrode 62 to which the gate electrode 6 a of the FET6 is connected (light detection means-to-gate connection step). Smallcharges generated at this time are released to the outside through theelectrode 63. It is thereby possible to prevent the destruction of theFET 6. Preferably, after this light detection means-to-gate connectionstep is executed, and before the next step is started, necessaryelements such as the capacitor and the resistor for the integrating readcircuit 4 are connected.

Gate-from-External Terminal Disconnection Step

The electrode 62, to which the electrode of the light detection means 3and the gate electrode 6 a of the FET 6 are connected, is disconnectedfrom the electrode 63 provided in the peripheral portion of thesubstrate 5 (in a gate-from-external terminal disconnection step). Themanufacturing of the photodetector for weak light according to thepresent invention is thus completed.

With this manufacturing method, the number of steps increases, ascompared with the conventional manufacturing method. However, since theFET 6 is connected to the light detection means 3 in an environment inwhich electric and thermal influences are eliminated, the photodetectorfor weak light according to the present invention can be manufacturedwithout damaging the FET 6.

According to the present invention, 500 nV√Hz@1 Hz is realized evenunder conditions in which the GaAs J-FET is used and the dielectricpolarization noise is generated. A response rate is 100 kilohertz. Evenat a gate input capacitance of 0.1 picofarad, if a single electron isinput, an output of 1600 nanovolts is obtained (see M. Fujiwara, M.Sasaki, and M. Akiba: “Appl. Phys. Lett.”, vol. 80, No. 10, 2002).Namely, an integrating read circuit 4 which has a S/N ratio of one ormore in response to the input of a single electron can be constructed.The gate leakage current of the GaAs J-FET is three electrons persecond.

A number-of-photons discriminator is provided in a liquid heliumcryostat. An optical system in which a single mode fiber is combinedwith a lens (a focal length of 4 millimeters, a condensed light diameterof 20 micrometers) introduces a signal light to the photodetector forweak light. This single mode fiber is fixed to be positioned just on thephotodetector for weak light by a fixing tool. The photodetector forweak light is arranged on a calcium fluoride substrate. Thephotodetector for weak light is manufactured as follows. A GaAs J-FET(D-mode, a junction width of 5 micrometers, and a junction length of 50micrometers) for an InGaAs PIN photodiode (a light reception diameter of30 micrometers) is fixed onto the calcium fluoride substrate patternedby Au and Cr, using a conductive paste. Respective element pads areconnected to patterned electrodes by metal wires each having a diameterof 25 micrometers. The substrate is formed as an XY stage through aframe fixing tool. The substrate is mounted to be distant from a ground(GND) of a work surface of the cryostat by 7 millimeters or more, andcan be also aligned to the optical axis. As the charge reset means, adevice in which a probe having a diameter of 7 millimeters is connectedto a motor, and which can be turned on and off by a rotational motion isused. To reinforce a rotational torque, a solenoid with a spring isconnected to the charge reset means. A tip end of the probe of thecharge reset means is connected to the GND. The electrodes on thecalcium fluoride substrate are introduced to and connected to a roomtemperature read circuit by constantan wires, respectively. A shield isapplied to entirely cover the XY stage, whereby it is possible toprevent a stray light from being made incident on the photodetector,prevent a radiation light, and cool the entire weak light detectionsystem to 4.2K.

The photodetector for weak light according to the present inventionoperates in an environment in which the photodetector is cooled to about4.2K. Conventionally, photodetectors for weak light do not operate at acapacity which can measure the number of photons unless it is cooled toabout 100 mK. The photodetector for weak light according to the presentinvention, by contrast, can measure the number of photons in thetemperature environment provided by liquid helium. The photodetector forweak light according to the present invention can be easily applied invarious fashions.

Further, since the photodetector for weak light according to the presentinvention can measure the number of photons, it can be utilized foroptical information technology, optical information processingtechnology, and the like.

Moreover, since the photodetector for weak light according to thepresent invention can detect a single photon, it can be utilized asexperimental device for quantum optics experiments.

1. A photodetector for weak light comprising: a substrate comprising anintegrating read circuit that includes light detection means and a fieldeffect transistor (FET); and charge reset means for releasing chargesaccumulated in the integrating read circuit, wherein the light detectionmeans is one of a PIN photodiode and an avalanche photodiode (APD), andthe charge reset means comprising: a rotation mechanism; a probe rotatedwhen the rotation mechanism rotates; and a connection section thatconnects the probe to an outside.
 2. The photodetector for weak lightaccording to claim 1, wherein the FET is GaAs JFET.
 3. The photodetectorfor weak light according to claim 1, wherein the integrating readcircuit is one of a capacitive trans-impedance amplifier (CTIA) circuitand a charge integrating amplifier (CIA) circuit.
 4. The photodetectorfor weak light according to claim 1, wherein the integrating readcircuit comprises: the FET; the light detection means connected to agate electrode of the FET; a capacitor connected to the gate electrodeof the FET; a resistance connected to a source electrode of the FET; andan operational amplifier connected to the source electrode of the FET.5. The photodetector for weak light according to claim 4, wherein acapacity of the capacitor is 0.01 to 1 pico farad (pF).
 6. Thephotodetector for weak light according to claim 4, wherein the capacitorhas an area of 0.1 to ten square millimeter (mm²), and a thickness of0.1 to 0.5 millimeter (mm).
 7. The photodetector for weak lightaccording to claim 4, wherein a dielectric that constitutes thecapacitor contains quartz glass.
 8. The photodetector for weak lightaccording to claim 1, wherein the integrating read circuit comprises:the FET; the light detection means connected to a gate electrode of theFET; a resistance connected to a source electrode of the FET; anoperational amplifier connected to the source electrode of the FETthrough the resistance; and a resistance that connects a negative inputterminal of the operational amplifier to an output terminal of theoperational amplifier.
 9. The photodetector for weak light according toclaim 8, wherein a resistance value of the resistances is 100 kilo ohms(kΩ) to 30 mega ohms (MΩ).
 10. The photodetector for weak lightaccording to claim 1, wherein the probe can be brought into contact withthe integrating read circuit, and can be separated from the integratingread circuit by 1 millimeter (mm) or more.
 11. The photodetector forweak light according to claim 1, wherein said charge reset means isseparate from said substrate.
 12. A photodetector for weak light,comprising: a substrate comprising an integrating read circuit thatincludes light detection means and a field effect transistor (FET); andcharge reset means for releasing charges accumulated in the integratingread circuit, wherein the light detection means is one of the groupconsisting of a PIN photodiode and an avalanche photodiode (APD), andwherein the charge reset means comprises: an ascent and descentmechanism; a probe that rises and falls by the ascent and descentmechanism; and a connection section that connects the probe to anoutput.